1. Field of the Invention
The present invention relates to a touch sensor unit used for a tester device for testing an electric circuit such as an integrated circuit and for detecting whether or not a prober for testing an electric circuit is in contact with the terminal of the electric circuit, and an electric circuit testing apparatus having the touch sensor unit.
2. Description of the Related Art
When a semiconductor element and particularly an IC (integrated circuit) is manufactured, 500 to 600 pieces of semiconductor chips are formed on a semiconductor wafer at the same time. After these chips are divided, the chips are contained in a predetermined package to complete an IC element. In this case, before the chips of the wafer are divided, electric characteristics of the respective chips of the semiconductor wafer are measured.
The measurements are conducted by contacting a plurality of testing probers, i.e., probe needles with a plurality of electrode pads of a semiconductor chip, respectively, and connecting the semiconductor chip to a measuring unit (tester) through the probe needles. In this case, a prober card on which a number of probe needles are provided is employed, and this prober card is mounted above a semiconductor wafer. The wafer is placed on a placing base movable in directions X, Y and Z. The base is moved to align the probe needles with the electrode pads. If the probe needles are aligned with the electrode pads, the base is raised in the direction Z (vertical direction), and the probe needles touch the electrode pads. At this time, the wafer is moved so that the wafer and the prober card are adjacent to each other, that is, the base is further raised at a predetermined distance from the height position where the probe needles touch the electrode pads (hereinafter merely referred to as "a touch position") so that predetermined needle pressure is applied to the probe needles to completely electrically contact the probe needles with the electrode pads. This movement from the touch position is called "an over-driver", and the moving distance is called "an over-drive distance".
Heretofore, the touch sensing has been carried out by a method for visually observing by an operator, an edge sensor for mechanically detecting the touch by a switch, or a needle sensor for detecting that a dc loop is closed when the probe needles are contacted with the electrode pads by supplying a dc current to the probe needles in contact with the street of the semiconductor chip.
When the touch sensing is carried out by an operator's visual observation, an irregularity occurs in the detected positions in accordance with the operator, and the same over-drive distance is detected differently in accordance with the needle pressure. Therefore, if the needle pressure is excessively applied, aluminum of the electrode pad is, for example, pierced by the probe needle to malfunction an IC. Particularly, if the IC chip has an high integrated density such as 4M, 16M and 64M, the electrode pad becomes as thin as the thickness of several micro-meters. As a result, the electrode pad is easily broken by the probe needle and thus the number of defective IC chips increases.
On the other hand, if the needle pressure is excessively low, a spontaneous oxide film formed on aluminum electrode is not pierced by the probe needle and the electrode pad is not in contact therewith, thereby disabling a correct measuring check of the IC chip.
Therefore, a method of detecting the touch, using an edge sensor or a needle sensor is considered, but according to the method of employing the needle sensor, if a circuit is formed in the street of the semiconductor chip, the touch sensing cannot be carried out due to an insulating film of the circuit.
According to the method of employing the edge sensor, the edge sensor is relatively expensive and a pressure must be applied to the sensor to mechanically detect the touch. Thus, a large irregularity occurs at the touch sensing position, and the method has a similar problem to that of the method for visually observing by an operator. Further, since it is necessary to apply a considerably high needle pressure to the probe needle so as to mechanically operate the edge sensor, there is an apprehension that the electrode pads, i.e., aluminum electrodes further reduced in thickness upon higher integration are pierced by the probe needles, and the method of employing the edge sensor is accordingly not practical.
Recently, the high integration of an IC chip is advanced to 32 Mbits and then 64 Mbits. In such a high integration IC chip, the electrode pads are considerably reduced to 30 to 60 .mu.m.
If a test for electric characteristics of the chip of a wafer is carried out in the state wherein its touch sensing is uncertain, even a normal chip is detected as being malfunctioned to deteriorate its yield. Therefore, in a semiconductor field, it is desired to develop a wafer prober which ensures a touch sensing.